Tunable RC filters are relatively new varieties of active continuous time filters and are useful for various low to medium frequency applications. Active RC filters are constructed from resistors, capacitors, and integrated amplifiers. For low to medium frequency applications, the amplifiers can be treated as having essentially infinite gain and input impedance. Because little or no current is drawn by the amplifiers, the amplifier inputs function as virtual grounds and substantially all of the input signal is applied to the resistors and capacitors. Thus, the operating characteristics of the filter are determined by the various RC products.
It is advantageous to be able to implement this type of filter as a monolithic integrated circuit. However, because of processing and temperature variations, the value of individual components may vary by as much as .+-.30%. It is not unusual for the RC product of on-chip components, and thus the frequency response of the filter, to vary by up to 50% from the nominal design value. One conventional solution has been to use integrated amplifiers in conjunction with high-precision, external resistors and capacitors. This solution increases both the size and the cost of the resulting filter.
Another solution is to provide for a fully integrated filter with an RC product that can be tuned to account for various process, temperature, and power supply variations. One method for doing this, is to replace passive resistors with active resistors made with MOSFET technologies. Y. P. Tsividis, "Integrated Continuous-Time Filter Design--An Overview, " IEEE J. Solid-State Circuits, vol. 29, pp. 166-176, March 1994. The filter is tuned by adjusting the resistance of the MOS resistors to achieve the desired RC value. For example, a feedback circuit may be implemented which continuously measures the RC product of the filter with reference to e.g., a clock or an external high-precision resistor. The outputs of the tuning circuit are then continuously applied to the MOSFET devices to set their resistances.
A disadvantage to this technique is that the tuning circuits are very complex, sometimes more complex than the filter itself. Another substantial drawback to this approach is that the inputs to the MOSFET resistors must be analog signals and thus must be continuously generated. The tuning circuit is therefore always drawing power. Further, this approach has serious drawbacks when low supply voltages of 2.4 volts to as low as 1 volt are used, a condition common in many battery operated devices. MOS transistors typically become conducting when a bias voltage of about one volt is applied. When low supply voltages are used, these devices do not have enough variable range to compensate for expected manufacturing variations of the filter components.
An alternative to this technique is to make tunable filters by using tunable capacitor arrays and linear, passive resistors in place of active, non-linear MOSFET resistors. A. M. Durham, J. B. Hughes and W. Redman-White, "Circuit Architectures for High Linearity Monolithic Continuous-Time Filtering," IEEE Transactions on Circuits and Systems--II:Analog and Digital Signal Processing, Vol. 39, No. 9, September 1992, pp. 651-57. The techniques proposed to tune capacitor arrays involve measuring the RC product of the array and comparing this value with the nominal design value. The number of active capacitors in the array is adjusted to keep the circuit near the nominal design for the RC product. Because passive resistors are used instead of MOSFET devices, the filter is highly linear. Furthermore, while the inputs to the MOSFET devices are analog, the inputs to the capacitor array are digital. Consequently, the tuning circuits that feed capacitor arrays generate digital outputs. Once the array has been calibrated, the resulting digital code can be latched while the rest of the tuning circuitry is powered down. The accuracy of the tuning process is necessarily limited by the number of bits in the digital code and consequently the number of controllable elements in the capacitor array. For many low frequency applications, an accuracy of .+-.5% to .+-.10% for the 3 dB point of the filter's frequency response is sufficient.
Conventional techniques for measuring the RC product involve using a dual-slope A/D convertor (see, e.g., Durham, et al., above) or switching and current mirror techniques. A. Durham and W. Redman-White, "Integrated Continuous-Time Balance Filters for 16-b DSP Interfaces," IEEE Journal of Solid State Circuits, Vol. 28, No. 7, July 1993, pp. 835-839. These tuning techniques are unnecessarily complex.
Thus, it is an object of the present invention to provide a simple and low-power tuning circuit which can calibrate a capacitor array to a high degree of accuracy.
Yet another object of the present invention is to provide a tuning circuit which can utilize electrical components of the filter to be tuned when the filter is not in operation.